CCNA Sunday - Spanning Tree Protocol and ARP Cache

CCNA Sunday - Spanning Tree Protocol

• Spanning Tree Protocol (STP) is a set of rules that layer 2 switches agree on and work with to prevent layer 2 loops in a network.
• Layer 2 loops occur when there are redundant paths in a network, causing packets to loop endlessly.
• STP is important to understand because it can save a person's bacon in a production environment if they're aware of why it behaves the way it does.

ARP Cache and Layer 2 Addresses

• ARP (Address Resolution Protocol) cache is a table that stores the layer 2 addresses associated with IP addresses on the same local network.
• The ARP cache can be viewed using the arp -a command.
• When a device tries to reach another device on the same network, it sends an ARP request to get the layer 2 address associated with the IP address.

Protocol Analyzer and Capturing Traffic

• A protocol analyzer is a tool that captures and displays network traffic.
• Wireshark is a popular protocol analyzer that can capture traffic on a local interface.
• Capturing traffic can help identify problems in a network, such as layer 2 loops.

Layer 2 Loops and Broadcasts

• A layer 2 loop occurs when a broadcast is sent on a network with redundant paths.
• The broadcast is forwarded by each switch, causing the packet to loop endlessly.
• This can be a problem because it can cause network congestion and redundancy.

Spanning Tree Protocol (STP)

• STP is a protocol that prevents layer 2 loops in a network.
• STP works by electing a "root bridge" switch, which is the central switch that all other switches communicate with.
• Each switch has a unique bridge ID, and the switch with the lowest bridge ID becomes the root bridge.

Bridge ID and Root Bridge Election

• The bridge ID is a unique number assigned to each switch.
• The bridge ID is used to elect the root bridge in an STP network.
• The switch with the lowest bridge ID becomes the root bridge.

Determining the Root Port

• Once the root bridge is elected, each switch determines its root port.
• The root port is the port that is closest to the root bridge.
• Each switch has only one root port.

Cost of Reaching the Root Bridge

• Each switch calculates the cost of reaching the root bridge.
• The cost is determined by the number of hops to reach the root bridge.
• The switch with the lowest cost becomes the root port.### Spanning Tree Protocol (STP)
• Purpose: prevent layer 2 loops in a network
• Operation:
  • Elects a root bridge based on the lowest bridge ID
  • Non-root bridges calculate their root port based on the lowest cost to reach the root bridge
  • Each segment has one designated port that forwards traffic away from the root bridge
  • Blocking occurs on ports that lose the election to be designated ports

Root Bridge Election

• Based on the lowest bridge ID
• Bridge ID consists of:
  • Priority (default: 32768)
  • VLAN number
  • Base MAC address

Root Port

• The port on a non-root bridge that has the best cost to reach the root bridge
• Always forwarding towards the root bridge
• Each non-root bridge has one root port

Designated Port

• The port on a segment that forwards traffic away from the root bridge
• Can be multiple designated ports on a switch
• Each segment has one designated port
• Always forwarding away from the root bridge

Blocking

• Occurs on ports that lose the election to be designated ports
• Prevents layer 2 loops in the network
• Blocking ports do not forward traffic

Spanning Tree Instances

• One instance per VLAN
• Use the command "show spanning-tree" to view all instances
• Use the command "show spanning-tree vlan " to view a specific instance

Verifying Spanning Tree

• Use the command "show spanning-tree" to verify the root bridge, root port, and designated ports
• Verify the bridge ID, cost, and port status on each switch### Spanning Tree Protocol (STP)
• Purpose: Prevents Layer 2 loops in a network
• Works by electing a root bridge and blocking redundant paths
• Uses Bridge Protocol Data Units (BPDUs) to communicate between switches

Root Bridge Election

• The switch with the lowest Bridge ID wins the election
• Bridge ID is a combination of priority, VLAN, and base MAC address
• If two switches have the same Bridge ID, the one with the lowest MAC address wins

Spanning Tree Operations

• The root bridge forwards on all its ports (designated ports)
• Non-root switches choose one port to forward on (root port) based on cost
• If costs are equal, the switch chooses based on Bridge ID
• If Bridge IDs are equal, the switch chooses based on advertised port ID

Tiebreaker Scenarios

• If a non-root switch receives BPDUs on multiple links with the same cost and Bridge ID, it chooses the link with the lowest advertised port ID
• This ensures that only one port is forwarding on each network segment

Verification and Troubleshooting

• Use show spanning-tree command to verify spanning tree topology
• Use show interface trunk command to verify VLANs allowed on trunk and being forwarded by spanning tree

Spanning Tree Configuration

• Use spanning-tree vlan root primary command to make a switch the root bridge
• This command lowers the priority of the switch to make it win the election

Spanning Tree Protocol (STP)

• STP is a set of rules to prevent layer 2 loops in a network.
• Layer 2 loops occur when there are redundant paths in a network, causing packets to loop endlessly.

ARP Cache and Layer 2 Addresses

• ARP cache stores layer 2 addresses associated with IP addresses on the same local network.
• The ARP cache can be viewed using the arp -a command.

Protocol Analyzer and Capturing Traffic

• A protocol analyzer is a tool that captures and displays network traffic.
• Wireshark is a popular protocol analyzer that can capture traffic on a local interface.

Layer 2 Loops and Broadcasts

• A layer 2 loop occurs when a broadcast is sent on a network with redundant paths.
• The broadcast is forwarded by each switch, causing the packet to loop endlessly.

STP Election

• STP elects a "root bridge" switch, which is the central switch that all other switches communicate with.
• Each switch has a unique bridge ID, and the switch with the lowest bridge ID becomes the root bridge.

Bridge ID and Root Bridge Election

• The bridge ID is a unique number assigned to each switch.
• The bridge ID is used to elect the root bridge in an STP network.

Determining the Root Port

• Once the root bridge is elected, each switch determines its root port.
• The root port is the port that is closest to the root bridge.

Cost of Reaching the Root Bridge

• Each switch calculates the cost of reaching the root bridge.
• The cost is determined by the number of hops to reach the root bridge.

STP Operation

• STP prevents layer 2 loops in a network by electing a root bridge and blocking redundant paths.
• STP uses Bridge Protocol Data Units (BPDUs) to communicate between switches.

Root Port

• The port on a non-root bridge that has the best cost to reach the root bridge.
• Always forwarding towards the root bridge.

Designated Port

• The port on a segment that forwards traffic away from the root bridge.
• Can be multiple designated ports on a switch.
• Each segment has one designated port.

Blocking

• Occurs on ports that lose the election to be designated ports.
• Prevents layer 2 loops in the network.

Spanning Tree Instances

• One instance per VLAN.
• Use the command "show spanning-tree" to view all instances.
• Use the command "show spanning-tree vlan " to view a specific instance.

Verifying Spanning Tree

• Use the command "show spanning-tree" to verify the root bridge, root port, and designated ports.
• Verify the bridge ID, cost, and port status on each switch.

Spanning Tree Configuration

• Use spanning-tree vlan root primary command to make a switch the root bridge.
• This command lowers the priority of the switch to make it win the election.